In the past, there have been developed a number of devices to measure the electrical parameters of a semiconductor. A typical semiconductor sample might have an oxide layer formed on top of a silicon layer. Additional oxide layers or polysilicon layers can be added on top.
In order to measure the electrical thickness of the layers in a non-contact fashion, a charge is placed on the sample. The capacitance is then measured. The tool can determine electrical thickness based on the equation:telectrical=∈A/C  (1)where t equals the thickness, ∈ is the electrical permittivity, A is the surface area and C is the capacitance. Permittivity ∈ is related to the index of refraction (n) by the formula ∈=n2 ∈0, where ∈0 is the permittivity of free space. Also of interest is the “dielectric constant” κ=∈/∈0 and the refractive index n2=κ.
One disadvantage of these electrical metrology devices is that the measurements can be skewed by factors that are either unobservable or are difficult to observe. For example, if the silicon substrate is contaminated with ions, this skews the voltage or capacitance measurements and, thus, the resulting calculations of electrical properties such as electrical thickness of the film.
Furthermore, the determination of at least some electrical properties of the film such as electrical thickness is dependent upon accurate knowledge of the dielectric constant or other inherent properties of the oxide layer. In a typical electrical metrology device, many of the parameters which cannot be measured are assumed to have a particular value. If this assumed value is not correct, the resultant determination of the characteristics of the sample will be incorrect.
Accordingly, it is an object of the subject invention to provide a system which provides multiple independent measurements in order to improve the accuracy of the evaluation of the sample.